This invention relates to the testing for the presence of native hydrocarbons down a borehole during drilling of the borehole.
It is conventional drilling practice, when hydrocarbon bearing levels are reached during the drilling of a well, to withdraw the drill string from the borehole and to run a corer down the borehole in order to withdraw a core of sample material from the bottom of the well for subsequent analysis. However, this is a time-consuming procedure and is very costly in terms of lost drilling time. In order to attempt to ensure that this procedure is not carried out erroneously before hydrocarbon bearing levels have been reached or that a hydrocarbon reservoir is not missed by coring too late, therefore, it is also the practice to collect samples of rock cuttings brought up from the vicinity of the drill bit by the circulating drilling mud flow, and to examine these in the presence of ultra-violet light. The aromatic components of native hydrocarbons flouresce strongly under ultra-violet light, that is they absorb light energy at a particular wavelength and emit light energy at a different, longer wavelength, and accordingly the particular nature of the fluorescence of the sample cuttings will indicate to a skilled observer whether the cuttings contain native hydrocarbons and thus whether the hydrocarbon bearing levels have been reached. In the event of a positive result of this test, the drill string may be withdrawn and the corer may be run down the borehole in order to confirm this result.
The above procedures have operated satisfactorily in the drilling of oil wells using conventional water-base drilling muds. However, such drilling muds are being replaced in a number of applications by oil-base drilling muds. The advantages of using oil-base drilling muds for drilling highly deviated wells in a number of fields have become widely recognised in recent years. It is possible to drill gauge holes quickly and without major problems because, among other attributes, these muds prevent hydration of shales, are stable in the presence of salts and at high temperatures and have excellent lubricating properties. As deeper and more problematic formations are investigated, oil-base muds are also becoming increasingly important in exploration drilling. Furthermore low toxic oil-base muds formulated with low aromatic base oils, instead of diesel as used in conventional oil-base muds, have been developed in response to government guidelines to the industry on the toxicity of discharges to the marine environment from offshore drilling rigs.
However, the use of diesel oil muds and low toxic oil muds alike presents difficulties in reliably testing for the presence of native hydrocarbons in the sample cuttings under ultra-violet light using the existing technique. The reason for this is that the oil base of the drilling mud also contains aromatic components which fluoresce under ultra-violet light. Under down-hole conditions mud filtrate to some extent invades the rock matrix and its presence confuses the detection of native hydrocarbons by the conventional method. The mixture of aromatic compounds contained in native hydrocarbons and drilling muds is complex, containing fluorescing species in the ultra-violet and visible parts of the spectrum. It is not unlikely that scattered and re-emitted light, which would be typically emitted in the ultra-violet at appropriate dilutions, may also be observed visually. Thus it may not be possible for an observer to distinguish between fluorescence due to the presence of the oil base of the drilling mud alone in the sample cuttings and fluorescence due to a combination of the oil base and native hydrocarbons. This is obviously highly unsatisfactory as it may lead to a false judgment that native hydrocarbons have been detected, resulting in a considerable loss of drilling time while the corer is run down the borehole, or alternatively may mean that a hydrocarbon reservoir is missed.
It is an object of the invention to develop a technique for reliably testing for the presence of native hydrocarbons during drilling with an oil-base drilling mud.